In a known delta modulator for digitally encoding analogue singals, a comparator compares an input analogue signal with a feedback signal derived from an integrator. The comparator provides an output which reflects the sign of the difference between the analogue input voltage and the integrator output voltage. This sign bit derived from the output of the comparator is sampled and fed to an output terminal as digitised data. The sign bit output obtained from the sampling circuit controls by means of a slope polarity switch, the direction of ramp in the integrator.
A demodulator for signals which have been digitally encoded using a delta modulator is essentially very similar to the circuit of the modulator the only differences being that the digitally encoded data is fed directly into the sampler circuit and demodulated analogue signals are obtained at the output of the integrator. There is therefore no comparator in the demodulator circuit.
Although the delta modulator circuit has the advantage of being very simple and provides digitally encoded data in serial format at its output, it is limited in its ability accurately to track the analogue input signal. Ideally the output of the integrator should accurately follow the input analogue signal.
An improvement to the basic circuit may be provided by using so called continuously variable slope circuitry which provides increased dynamic range by adjusting the gain of the integrator.
The output of the sampler circuit is monitored and in dependence upon this output the ramp current fed to the integrator is increased or decreased in order to produce better tracking of the output of the integrator. In one form of continuously variable slope circuitry the output of the sampler circuit is monitored to determine the occurence of a predetermined number of one or zero bit outputs, typically a string of three or four ones or zeros. A typically known continuously variable slope delta modulator/demodulator of this type is the Motorola MC3517 integrated circuit.
Although the use of a continuously variable slope circuit in a delta modulator/demodulator provides improved performance and dynamic range the modulator/demodulator may still respond too slowly to follow fast rising and falling waveforms and high frequencies.
It is known to use resistor-diode networks in order to increase the ramp voltage fed back to the integrator but for many applications such networks do not give adequate tracking performance. It is not possible to obtain smooth control of the slope of the ramp in the integrator, the diode-resistor networks providing a stepwise adjustment. Also since the characteristic of diodes is strongly affected by temperature the correction provided by the diode-resistor networks is also strongly temperature dependent. This may result in relatively poor fidelity of the reconstructed waveform at the output of the integrator.
This invention seeks to provide a delta modulator/demodulator in which at least some of the above mentioned disadvantages are mitigated.